The present invention relates to recording encoded motion picture video data, and is particularly applicable to editing coded motion picture data which has been obtained by high efficiency coding of video signals.
Heretofore, there has been proposed a recording/reproducing system in which video signals which represent a moving picture image are high efficiency encoded as intraframe coded data and interframe coded data. This encoded data admit of high density, and are recorded on a compact disk, such as a magneto-optical compact disk (CD-MO disk), which may be searched and reproduced when desired (see, for example, Japanese Patent (unexamined) Publication No. 63 (1988)-1183 and Japanese Patent Application No. 1 (1989)-267049).
More specifically, for example, as shown in FIG. 1, each image frame PC1, PC2, PC3, . . . which are motion picture imaged at respective times t=t1, t2, t3, . . . is digitized and then sent to a transmission system including CD-MO recording/reproducing system, for example. Processing is performed to enhance transmission efficiency by compressing the image data to be transmitted. Data compression relies upon the fact that video signals exhibit high autocorrelation over time. In intraframe coding, image differences are obtained by comparing pixel data in a image frame with reference data and for each of the image frames pC1, pC2, pC3, . . . , in which intraframe coding is carried out, image data is sent, the image data having been compressed using autocorrelation between pixel data in the same frame.
For interframe coding, differences in the pixel data between adjacent images PC1 and PC2, PC2 and PC3, . . . , respectively, are calculated to produce image data PC12, PC23, etc. This interframe encoded image data are sent together with the intraframe encoded image data obtained from the initial image frame PC1 at the time t=t1.
It is thus possible to code the images PC1, PC2, PC3, . . . with high efficiency to provide a reduced amount of digital data as compared to the case where all of the image data is sent. The compressed data then may be recorded on a CD-MO recording/reproducing system.
The encoding of such video signals is carried out in a coded motion picture data generating unit 1 having a configuration shown in FIG. 2.
In the coded motion picture data generating unit 1, conventional processing techniques are used on input video signals VD, such as every other field dropping, one field line thinning, and the like, in a preprocessing circuit 2, and luminance signals and chrominance signals of the processed video signals are then converted to transmission unit block data S11 (hereinafter referred to as a macro block). The macro block data S11 is comprised of data representing 16 pixels (horizontal).times.16 pixels (vertical) and is fed to an image data coding circuit 3.
The image data coding circuit 3 receives predictive current frame data S12 produced by a predictive coding circuit 4 (which is conventional) and calculates the difference between that data and the macro block data S11 to thereby generate interframe coded data (this is referred to as the interframe coding mode). This interframe data is fed to a transform coding circuit 5 as differential data S13. Image data coding circuit 3 also operates to produce intraframe encoded data by calculating the difference between the macro block data S11 and reference data, as is conventional. In the intraframe coding mode the intraframe data is supplied as differential data S13 to the transform coding circuit 5.
The transform coding circuit 5 consists of a discrete cosine transform circuit, and, as is known, performs high efficiency coding on the differential data S13 by orthogonal transformation to produce transform coding data S14. The transform coding data S14 is supplied to a quantization circuit 6, which produces quantized image data S15.
The quantized image data S15 which has been thus obtained from the quantization circuit 6 undergoes further high efficiency coding in a retransform coding circuit 7 which includes a variable length coding circuit, and is then supplied as transmission image data S16 to a transmission buffer memory 8.
In addition to this, the quantized image data S15 is subjected to inverse quantization and inverse transform coding in the predictive coding circuit 4, and is thereby decoded to produce the predictive current frame data S12 which is compared to the macro block data S11 to generate the differential data. The predictive coding circuit 4 corrects predictive previous frame data as a function of the differential data, and thereby stores new predictive previous frame data. Furthermore, the predictive previous frame data which is stored in the predictive coding circuit 4 is motion compensated by motion detection data produced from the macro block data S11 to thereby produce predictive current frame data, which is fed to the image data coding circuit 3. Thus, the difference between the macro block data S11 of a frame to be currently transmitted (i.e., the current frame) and the predictive current frame data S12 is obtained as the differential data S13.
If the motion picture images described with reference to FIG. 1 are processed by the unit of FIG. 2, the image data of the image frame PC1 is firstly provided as the macro block data S11 at a time t1 (FIG. 1) and the image data coding circuit 3 is placed in the intraframe coding mode. Hence, intraframe coded differential data S13 is supplied to the transform coding circuit 5, and intraframe transmission image data S16 is supplied to the transmission buffer memory 8 via the quantization circuit 6 and retransform coding circuit 7.
Concurrently, the quantized image data S15 obtained at the output of the quantization circuit 6 undergoes predictive coding in the predictive coding circuit 4, and the predictive previous frame data which represents the transmission image data S16 sent to the transmission buffer memory 8 is thereby held in the previous frame memory. When macro block data S11 which represents the image PC2 at t=t2 is subsequently fed to the image data coding circuit 3, it is interframe encoded (or motion compensated) by using the predictive current frame data S12.
Thus the image data coding circuit 3 supplies interframe coded differential data S13 to the transform coding circuit when image frame PC2 is encoded and thereby differential data which represents a shift of the image between the frames PC1 and PC2 is fed as transmission image data S16 to the transmission buffer memory 8. Concurrently, the predictive previous frame data is produced and stored in the predictive coding circuit 4 by supplying the quantized image data S15 to the predictive coding circuit 4.
While interframe coding is used for successive image frames, only differential data which represents the shift of the image between the previous frame and the current frame is sequentially sent to the transmission buffer memory 8 by repeating the foregoing operations.
The transmission buffer memory 8 temporarily stores the transmission image data S16 which is sequentially read and sent as transmission data DTRANS Via a transmission line 9 (for example for recording) at a data transmission rate which depends on the transmission capacity of the transmission line 9.
At the same time, the transmission buffer memory 8 detects the amount of the data therein remaining to be transmitted, and feeds back an indication S17 of the remaining data amount to the quantization circuit 6 to control the quantization step size according to this remaining amount indication S17. Thus, an appropriate amount of remaining data (which will not produce overflow or underflow) is kept in the transmission buffer memory 7 by adjusting the amount of data which constitutes the transmission image data S16.
If the remaining amount of data in the transmission buffer memory 8 reaches an upper allowable limit, the step size of the quantization step STPS (FIG. 3) of the quantization circuit 6 is increased so that coarser quantization is carried out in the quantization circuit 6 to thereby decrease the amount of data which constitutes the transmission image data S16.
On the other hand, if the amount of data remaining in the transmission buffer memory 8 decreases to a lower allowable limit, the step size of the quantization step STPS of the quantization circuit 6 becomes smaller, whereby the amount of data which constitutes the transmission image data S16 is increased because the quantization circuit 6 carries out finer quantization.
It will be appreciated that the coded motion picture data generating unit 1 operates to produce compressed moving picture data DTRANS in accordance with ISO standard 11172 comprised of intraframe coded frames A1, A9, . . . (hereinafter referred to as intraframes and indicated by the character "A"), interframe coded previous frame prediction coded frames B3, B5, B7, . . . (hereinafter referred to as predictive frames and designated by the character "B"), and interpolated prediction coded frames C2, C4, C6, . . . (hereinafter referred to as interpolated frames and indicated by the character "C") representing the sequence of input image frames of video data VD as shown in FIGS. 4A and 4B.
According to the ISO standard, when this transmission data DTRANS is received or reproduced and the image frame corresponding to the interpolated frame C2, for example, is to be recovered, the intraframe A1 and the predictive frame B3 are needed as shown in FIG. 4C to decode the interpolated frame C2. Hence, a memory and a memory control circuit are needed to decode this encoded motion picture data. That is, the memory is needed to delay the interpolated frame C2 until the intraframe A1 and the predictive frame B3 are received. This makes the decoder circuit configuration rather complicated and excessively enlarges the amount of decoder delay.
For this reason, the Moving Picture Expert Group (MPEG) of the ISO has proposed that the transmission data DTRANS be reordered before transmission (or recording) as shown in FIG. 4C to facilitate the decoding operation. The reordered transmission data DTRANS is recorded in groups of frames GOF1, GOF2, each of which is formed of 8 frames (A1, C2, B3, C4, B5, C6, B7 and C8) between intraframes A1 and A9, . . . and each group of frames GOF is recorded in 20 sectors, for example, of the CD-MO disk.
However, if an edit operation is carried out to replace group of frames GOFI, for example, recorded on a CD-MO disk with a new group of frames, it is not possible to reproduce video signals accurately from the new group of frames because the interpolated frame C8 which had been reordered and recorded in the old group of frames GOF2 had been part of the old group of frames GOF1 and, as shown in FIG. 4A, when frame C8 is decoded it will be interpolated on the basis of the new seventh predictive frame B7 in the new group of frames GOF1 and the old first intraframe A9 in the group of frames GOF2. But, frame C8 had been recorded prior to editing by interpolating the old seventh prediction frame B7 of the old group of frames GOFI and, thus, the old seventh predictive frame B7 (which had been replaced by the edit operation) is needed to decode frame C8. As a result, there is a problem in that the edited video data DTRANS is not correctly reproduced.